Multiplex signaling



Nov. 16 1926. 1,606,795

K. s. JOHNSON El AL MULTIPLEX SIGNALING Filec} Jan. 11 1924 2Sheets-Sheet 1 P o $1 -\9mmm;-

H POF Patented New. 16, 1926.

UNITED STATES PATENT orrlcs.

KENNETHS. JOHNSON, OF JERSEY CITY, AND MAURICE B. LONG, OF GLEN RIDGE,NEW JERSEY, ASSIGNORS TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEWYORK, N. Y., A CORPORATION DIE-NEW YORKQ MULTIPLEX SIGNALING.

This invention relates to multiplex signaling and particularly tofiltering system for use therein.

It is an object of the invention to provide a new and improved means forcompensating for the unequal attenuation of the dif: ferent waves in thetransmission medium of a multiplex signaling system.

Another object-of the invention is to provide means whereby thefrequency spacing required between oppositely directed groups offrequencies is materially reduced, thereby rendering availableconsiderable frequency space for additional communication channels.

In multiplex signaling systems in which the transmitted energy is in theform of waves or oscillations modulated in accordance with the signalsto be transmitted, a plurality of such waves, each of a distinctivefrequency, are transmitted over the same line. Due to theinherentattenuation of the transmission line and its associated apparatus,however, it is necessary to amplify these oscillations. This may beaccomplished by providing a repeater system in which oppositely directedwaves are transmitted at different frequencies, the repeater being soarranged that allof the wavestransmitted in one direction will beamplified by a single one-way repeater and all of the waves transmittedin the opposite direction will be amplified by another one-way repeater.

In such systems, particularly those employing long .line conductors, itis well known that the waves of higher frequencies are less perfectlytransmitted than those of lower frequencies. The'wider the range offrequencies transmitted the greater is the distortion due to the poorertransmission or greater attenuation of the high frequencies relative tothe low.

' If a repeater of the above mentioned type is used for a large range offrequencies on a line possessing unequal attenuating properties, it isdesirable to assoc ate with t e repeater somemeans for equaliz ng orcompensating for the distortion due to such attenuation. In general, itis desirable to introduce the means of correction for the lineattenuation at the repeater since the correctin means introduces a losswhich may be ma e up by the repeater. Heretofore, t9

meet these requirements, attenuation e ualizers and filters have beenemployed w ich have. each been separately designed with respect to theirarticular functions, and

in practice considhrable frequency margin,

has been thought necessary between the 0ppositely directed carrierwaves.

In accordance with a. feature of the invention a carrier signalingsystem is provided with a repeater comprising grouping filters toseparate oppositely directed groups of frequencies, and attenuation Thegrouping filters each comprise a plu rality of sections separated by anampllfier which thus divides the sections into two groups. Theattenuation equalizers are so designed that when combined with thefilters and the line, the transmission efliciency of the entirecircuitis essentially constant over the transmitted range offrequencies. The combined filter and equalizer then act as a filterhaving attenuation characteristics equalizers.

over the transmission range complementary to those of the transmissionline, and at the same time having high attenuation over the attenuatedrange. By this arrangement, the frequency spacing required betweenopposit'ely directed groups of frequencies can be materially reduced.

Other features and advantages of the invention will be apparent from theconsideration of the following description taken in connection with theaccompanying drawing, in which: i

Fig. 1 is a diagrammatic illustration of a two-way amplifying repeater,connecting two line sections of a carrier transmission line, constructedin accordance with the invention.

Figs. 2 and 3 are curves illustrating the attenuation of the variousparts of the system of Fig. 1.

The system shown in Fig. 1 is particularly adapted for multiplex carrierwave transmission in which a plurality of modulated carrier waves, eachof which represents an individual message, may be simultaneouslytransmitted. Thetransmissions in the .two directions are obtained bycurrents of different frequency ranges. For example, line section W maysupply only currents of frequencies higher than 6,000 cycles to sectionE, and section E in turn may transmit only currents of frequencies,lower than 6,000 cycles to section W. The two repeating paths orbranches 4 and 5 of the repeater are each equipped with a filter tosuppress currents of the fre uencies transmitted by the other branch, tus effectively preventing singing.

Each branch'et and 5 includes an amplifier A, preferably of the highlyevacuated three element-electron discharge type, for amplifying thecurrent supplied to that branch. 'In order to secure faithfulrepetition, these" amplifiers and their circuits are designed in wellknown manner to produce as little intermodulation'or other distortionofthe different frequency currents as possible. Thebranch 4 also includesa high pass input filter. section HPIF, an attenuation equalizer AE anda high pass output filter section HPOF, while the branch 5 includes alow pass input filter section LPIF, an attenua tion equalizer AE and alow pass output filter section LPOF. In accordance with the inventiondisclosed in Patent No. 1,413,- 357 to P. A. Raibourn, April '18, 1922,the amplifiers A are interposed between the in put and output. filtersections in each ofthe branches so that part of the filter sections are"connected to the input circuit ofthe closed in British Patent No.186,198, and

may be of the composite wave filter type as discussed in part 3 of anarticle on the Theory and design of'uniform and composite electricwave-filters, by Otto J. Zobel, in the Bell System Technical Journal ofJanuary, 1923. These filters consist in gen eral of a plurality ofsections having series and shunt reactances designed, according to wellknown laws, for the range of frequencies of the currents which thefilters are to transmit. Filters of this type are known as suppressionfilters and are particularly advantageous where asharp cut-off betweenfrequencies in the transmitted and suppressed range's is desired. Thefilter section HPIF is of the high pass type and is adapted to suppressfrequencies below a definite limit, while filter section LPOFis of thelow-pass type and is adapted to suppress frequencies above a definitelimit.

Since the attenuation of a transmission line is a function of thefrequency, the carrier currents at the receiving end of the line willvary in magnitude corresponding with the attentuation of the line whichthey have traversed. Where these currents are rede igned peated into asucceeding line the variation in magnitude at the receiving end of thesecond line is proportionatel much greater than for the firstline, andcups equalizers have been devised for use at repeater pointls 'whicheffectually decrease the amplification of the repeater for the lowerfrequencies. In the present system the equalizers AE are 1ncludedin therepeater network between the amplifiers A and the input filters HPIF andLPIF, respectively. The addition of these equalizers introduces a lossin the-transmission range of the filters in repeater paths 4;, and 5.

If curves and ll of Figs. 2 and 3, representing the characteristicsofthe high and low pass filters, respectively, were each extendedvertically to the'fr'equency axis, it

would be possible by means of the grouping filters to separateoppositely directed frequencies' which approached-within few cycles ofeach other.- In practice, theremust always be left a substantial'frequencymargin between oppositely. directed transmissions on accountof the gradual cut-ofi' --whi ch a filter has and which is indicatedparticularly by the curved lower portions-of the characteristics 10 and11. Even-if,..for;instance. the filter cut-offs were made so-close toeach other that the lower curved portions of characteristics 10 "and 11overlapped, there would still be aregion common to the transmissionranges of both filters within which the filters would be .inefifectualtoseparate the frequencies and this ;frequen y interval couldnot-:therefore'- be utilized.

In accordance with the inventiomtheat: tenuation equalizer is made tohave a characteristic which combines, with the lower curved portion ofthefilter characteristic l0 or 11 to give a resultant characteristic foreach repeater path comprising an attenuation equalizing portion and afrequency suppression portion, with such a sharp cutoil between the twofrequency regions that the practical, frequency margin necessary to beleft between the oppositely directed transmission is materially reduced.H

The attenuation equalizers. AE are ,preferably of the type which have aconstant resistance characteristic impedance as disclosedin anapplication of. O.' ,-J. Zobel,

Serial No; 580,7 69, filedAugust 9', 19.22, ,according to which anetwork of this type having any desired. attenuation-frequencycharacteristic may be designed. In the present case, the filter sectionsHPIF and HPOF of branch 4 are designed to pass currents of frequenciesabove about 6,000 cycles and to effectively suppress 'currents offrequencies below 6,000 cycles, as represented by the characteristiccurve 10 of-Fig 2.. 2.1116 filter sections LPIF, tindLPOE .of branchfijiire Y to pass currents lfof frequencies below 6,000 cycles andto'efiectively suppress currents of frequencies above 6,000 cycles asattenuation equalizer AE "of branch 5 are represented by the curve 13 ofFig. 3. The characteristics of this equalizer when combined with thecharacteristic of the network represented by the curve 11 will give anover-all characteristic represented by the full line portion of thestraight line 15. In order that the attenuation equalizer of branch 4may possess the characteristic represented by the curve 12, it isevident that the attenuation of this equalizer must be substantiallyzero at .two points, namely at 6,400 cycles and at 10,000 cycles.- Thegeneral method of. constructing such an equalizer is described in theabove-mentioned Zobelapplication. If then, one of the series arms of theequalizer is made resonant at 6,400 cycles and. one of the shunt arms ismade, anti-resonant at 6,400 cycles, the series arm will offer zeroimpedance and the shunt arm will offer infinite impedance to the passageof currents of this frequency, thus causing no attenuation for currentsof 6,400 cycles. The other series arm is made resonant at 10,000 cyclesand the other shunt arm is made anti-resonant at 10,000 cycles, thuscausing no attenuation for currents of 10,000 cycles. The seriesresistance arm of the equalizer is of such a value as to determine themaximum loss given by the equ-al-' izer, or in other words to determinethe peak of the curve 12. The attenuation equalizer of branch '5 maylikewise be designed to give the characteristics represented by thecurve 13. i v

. While the invention has been shown and described in connection with amultiplex carrier transmission system, it will be understood that it isnot to be limited to such a system.

It will also be obvious that the general principles herein disclosed maybe embodied in many other organizations widely different from thoseillustrated without departing from the scopeand spirit of the inventionas defined in the following claims.

What is claimed is:

1. In a wave transmission system employing Waves comprised in differentfrequency ranges, a line the attenuation of which. differs for differentfrequencies, filters for separating oppositely directed groups offrequencies, and means for compensating for' the distortion due'toattenuation, said compensating means including networks each having acharacteristic which corrects for the non-linear characteristic of arespective filter over part of the range of the transmitted frequencies,and said compensating means and said filters having combinedcharacteristics which correct for the nonuniform attenuation of saidline.

2. In a wave transmission system employing waves comprised in differentfrequency ranges, a line the transmission loss of which differs fordifferent frequencies, means for separating oppositely directed groupsof frequencies, and means for compensating for the distortion due to theVariable transmis sion loss, said separating and compensating meanshaving combined loss characteristics over the transmission range complementary to those of the transmission line, said separating means andsaid compensating means each having by itself a loss characteristicmaterially different from a loss characteristic complementary to that ofthe line throughoutat least a portion of the frequency range over whichtheir combined transmission loss is complementary to that of the'line. Iv i 3. In a wave transmission system employing waves comprised indifferent frequency ranges, a line the transmission loss of whichdiffers for different frequencies, and a network comprising groupingfilters to separate oppositely directed groups of frequencies andattenuation equalizers, said network having characteristics over thetransmission range complementary to those of the transmission line andhaving high attenuation over the suppression range, said groupingfilters and said attenuation equalizers each having by itself a losscharacteristic materially different from a loss characteristiccomplementary to that of the line throughout. at least a portion of thefrequency range over which their combined transmission loss iscomplementary to that of the line.

4. A wave transmission system employing waves comprised in differentfrequency ranges, a line the transmission loss of which differs for d1fferent frequencies, and a repeater 1n sald line including means forseparating oppositely directed groups of fre quencies and an attenuationequalizer, said separating means and said attenuation equalizer havingcombined transmission characteristics over the transmission rangecomplementary to those of the transmission line, said separatin meansand said attenuation equalizer each1aving by itself a losscharacteristic materially different from a loss characteristiccomplementary to that of the line throughout at least a portion of thefre quency range over which their combined transmission characteristicis complementary to that of the line.

5 Awave transmission'system employing waves comprised in differentfrequency ranges, a line the transmission loss of which differs fordifferent frequencies, and a network comprising means for amplifying allof said waves substantially uniformly, grouping filters to separateopposite directed groups of-- frequencies and attenuation equalizers,said network having characteristics over the transmission rangecomplementary to those of the transmission line, said grouping filtersand sa-id attenuation equalizers each having b itself a losscharacteristic materially di erent from a loss characteristiccomplementary to that of the line throughout at least a portion of thefrequency range oyer which their combined transmission loss iscomplementary'to that of the line.

6. In a wave transmission system employing waves comprised in differentfrequency ranges, a. line the transmission loss of which differsfordifferent frequencies, a filter for freely transmitting waves comprisedin a certain frequency range and for suppressing the transmission ofwaves of frequencies.

outside said range, the attenuation of said filter varying over anappreciable frequency interval in a manner markedly different from thatof said line, and van attenuation equalizer associated with said filter,said attenuation equalizer and said filter having a combinedtransmission characteristic complemental to that of the line throughoutthe range of free transmission of said filter and a said attenuationequalizer having a transmission characteristic which varies in a mannerto compensate for the dilference in variation of the attenuation of saidfilter and said line over the said appreciable frequency Interval.

7. In a wave transmission system, em- 4" ploying waves comprised indiiferent frequency ranges, a filter for freely transmitting wavescomprised in a certain frequency range and for suppressing thetransmission of waves of frequencies outside that range, said filterhaving a non-linear transmission loss characteristic in the neighborhoodof the cut-off frequency, and means for compensating for the nonlinearloss charac teristic of said filter in the neighborhood of ationequalizer having a transmission loss characteristic which substantiallycorrects for the said no'nlinear portion of the characteristic of saidfilter in the neighborhood of the cut-off frequency In witness whereof,we hereunto subscribe our names this 10th day of January 1).,

KENNETH s. JOHNSON. MAURICE B. LONG.

